Knowledge of individual cylinder values of Indicated Mean Effective Pressure (IMEP) is known in the prior art as a powerful tool for evaluating and correcting poor combustion in an internal combustion engine. By definition, IMEP, in kiloPascals, is defined as the ratio of the indicated work in Newton meters (W) divided by the swept volume per cylinder (V) in liters:IMEP=W/V  (Equation 0)
IMEP is an accepted standard method for measuring combustion in internal combustion engines. The information is valuable in indicating combustion quality and is used extensively in the prior engine arts in engine dynamometer work to characterize and quantify acceptable and unacceptable combustion performance. IMEP is known to be used to determine the limits of engine dilution (e.g., exhaust gas recirculation, camshaft phasing), spark advance angle, and rich/lean limits to engine fueling.
Although IMEP is a valuable parameter for combustion development, its use in real time engine controls has been limited in the prior art in general because its determination has required expensive and non-durable combustion analysis equipment, and because the prior art methods of measurement have been engine-intrusive (e.g., combustion pressure sensors in the engine heads or spark plugs). Other known methods of combustion quality measurement, such as Ion Sense technology, require expensive hardware upgrades and have not been generally available. Off-board rack-type analysis equipment is bulky, expensive, and non-portable. Thus, engine control using IMEP has been largely a laboratory phenomenon rather than being useful day-to-day in an operating vehicle.
Less than ideal combustion performance can arise from a variety of sources including: engine component design limitations; variations in fuel properties in the field; aged engine components; and manufacturing tolerances of engine subassemblies and components. Manufacturing tolerances for valve train intake and exhaust ports and valves; fouled plugs, ports or injectors; and/or design trade offs affecting fuel, purge, PCV and EGR distribution, can all contribute to degraded combustion quality. Contributors to degraded combustion can affect performance of individual cylinders, or of the engine as a whole.
An individual cylinder torque estimator, used in conjunction with an appropriate engine algorithm in real time control, can mitigate the sources of cylinder-to-cylinder combustion variability, ultimately improving, for example, idle quality, NVH due to torque imbalance, peak power, and cold start emissions.
Prior art methods which attempt to estimate individual cylinder torque values focus on assessing combustion performance based upon a single cycle or single-cylinder event. When attempting to evaluate combustion quality, quantifying only single-cylinder events can be misleading due to cyclic variability of fuel transients in the ports, or to unburned fuel residuals which remain after partial burns or misfires. Incomplete mixing and burn due to in-cylinder turbulence which is unrepresentative of overall combustion behavior may also result in poor combustion on a single cylinder event basis.
Using a statistical evaluation of IMEP as a metric to gauge combustion quality is therefore advantageous and superior. The Coefficient of Variance of IMEP (COVIMEP) is a statistical evaluation of combustion quality. COVIMEP is a way of characterizing engine combustion that is well accepted across the automotive industry. As such, it provides an objective and standard means for quantifying combustion performance. Because of its ready availability, correlation to other engine performance characteristics, for example, brake-specific emissions values, is also possible.
In addition to the lack of a good metric for evaluating combustion quality that can be used in real time control, prior art methods have also required additional development effort to calibrate their models. While such development effort is of value for improving the model's accuracy, it provides limited additional benefit beyond the express purpose of individual cylinder torque estimation.
Further, depending on complexity, prior art methods can be computationally expensive which limits their use, especially at high engine speeds when the chronometric impact of calculations which must be performed in the period between cylinder firing events, i.e. calculations for individual cylinder torque estimation, is greatest.
What is needed in the art is a method for providing cylinder IMEP information, and an associated control metric, that does not require additional engine hardware or significant development effort and computational expense, while at the same time providing good utility for real time engine control.
It is a principal object of the present invention to provide realtime IMEP and COVIMEP for each cylinder of a multi-cylinder engine, and the engine as a whole, from calculated crankshaft velocities and accelerations, and from a pre-existing algorithm which requires little or no additional calibration effort for the present purpose. Additionally, the present invention includes calculations which are simplified and optimized for computational efficiency and speed.